The key performance parameters for optical fibers are attenuation (e.g., energy losses), dispersion, and mode-field diameter. Attenuation is the reduction of signal strength or light power over the length of the light-carrying medium. As a consequence of attenuation, a transmitted signal weakens and must be strengthened or repeated. Fiber attenuation, which is measured in decibels per kilometer (db/km), varies as a function of wavelength.
Attenuation specifications for the telecommunications industry have become increasingly rigorous. As compared to other transmission media (i.e., copper, coaxial cable, etc.), optical fiber requires low attenuation, typically 0.35 dB/km at a wavelength of 1385 nanometers for standard single-mode fiber (SSMF). Attenuation requirements at a wavelength of 1550 nanometers are even lower, typically 0.25 dB/km. Low attenuation provides a transmitted optical signal the ability to travel more than 100 kilometers without regeneration or amplification.
Attenuation in optical fibers can be caused by several different factors but is mostly a result of scattering and absorption. The scattering of light from molecular-level irregularities in the glass structure leads to the general shape of the attenuation curve, called Rayleigh scattering. Further attenuation is caused by light absorbed by residual materials (e.g., metals or water ions) within the fiber core and inner cladding.
The strong absorption at a wavelength of 1385 nanometers is due to hydroxyl groups present in the optical fiber. For instance, U.S. Patent Publication No. 2005/0120751 A1, which is hereby incorporated by reference in its entirety, discloses that, during the formation a glass preform during a chemical vapor deposition (CVD) process, hydrogen atoms in the glass form OH— bonds in the glass layers, thus adversely affecting the transmission spectrum of an optical fiber drawn from the preform, particularly at wavelengths of 1240 nanometers and 1385 nanometers.
U.S. Pat. No. 3,782,914, which is hereby incorporated by reference in its entirety, discloses a process for heat treating optical waveguides, which include a core of doped silicon dioxide (SiO2) and a cladding of doped or undoped silicon dioxide (SiO2), at 700° C. to 1200° C. for less than six hours. Such heat treatment purportedly oxidizes the reduced dopant oxide, which in turn reduces attenuation.
Furthermore, it is believed that transition metals and their interactions with the vacancies and impurities affect the wavelength and the absorption coefficient in optical fiber. For example, U.S. Pat. No. 7,092,610, which is hereby incorporated by reference in its entirety, discloses adding controlled amounts of transition metals (i.e., cobalt, nickel, chromium, vanadium, iron, manganese, thallium, and thulium) as dopants in an optical fiber to attenuate optical signals (e.g., in an optical attenuator or optical terminal device).
To reduce attenuation at a wavelength of 1550 nanometers, U.S. Pat. No. 6,952,516, which is hereby incorporated by reference in its entirety, discloses a hydrogen-proof treatment method, namely holding optical fiber in a treatment tank charged with D2 (heavy hydrogen) for an extended period (e.g., three hours or more).
U.S. Pat. No. 6,799,440, which is hereby incorporated by reference in its entirety, suggests a method for improving attenuation in optical fiber by treating the quartz deposition tube that is used to make the glass preform, which itself is subsequently fabricated into optical fiber. This patent discloses a process in which silica tube is fused in a D2 (heavy hydrogen) atmosphere to achieve decreased fiber attenuation. Alternatively, this patent discloses that the deposition tube can be formed from silica sand pretreated in a D2 gas atmosphere prior to fusing the deposition tube.
Despite the foregoing, there is still a need for high-quality optical fiber that meets the rigorous demands of the telecommunication market. In particular, there is a corresponding demand for high-quality glass deposition tubes that facilitate the formation of optical preforms and optical fibers that employ reduced amounts of inner cladding material.